1. Field of the Invention
The present invention is directed to photoresist compositions and more particularly to photoresist compositions containing cross-linked polyalkenyl phenols as the polymeric binder.
2. The Prior Art
The formation of resist masks in the manufacture of integrated circuits using radiation sensitive films which contain a film-forming thermoplastic polymeric binder mixed with a radiation sensitive compound is known.
The use of such materials in integrated circuit technology is based on their property to permit the "engraving" of circuit images of predetermined dimensions on a monolithic substrate material such as silicon. Layers of the radiation sensitive material are formed on the substrate and exposed to light or other activating radiation such as electron beam or x-rays in a patternwise manner using a mask corresponding to the circuit design. By means of a subsequent developing process, the desired resist patterns are formed on the substrate surface. Thereafter, the substrate is subjected to an additive or subtractive process, such as metallization or etching, with the remaining portions of the resist layer acting to protect the substrate.
According to their interaction with the activating radiation, a distinction is made between negative and positive photoresist systems. The term negative refers to a photoresist which after exposure in a suitable solvent is insoluble, whereas the unexposed resist zones are dissolved by the developer. As a result, free and unprotected zones are obtained on the substrate surface which correspond to the opaque dark areas of the photomask. In a positive working photoresist system, the photoresist is altered upon exposure in such manner that it is subsequently soluble in the developer (e.g. aqueous buffered alkali). The exposed areas of the photoresist film are removed upon developing, and the free unprotected areas on the substrate correspond to the transparent parts on the photomask.
In U.S. Pat. No. 4,104,070, it is disclosed that when a positive photoresist system is modified with a 1-hydroxyethyl-2-alkylimidazoline and heated, after image-wise exposure, for a sufficient period of time to a sufficient temperature to render the exposed areas insoluble in alkaline developer and then blanket exposed with actinic radiation, a negative photoresist image is obtained. If there is no thermal treatment and blanket exposure after the image-wise exposure, a positive image is obtained from the same mask.
Among the polymeric resins which have been considered for use as photoresist materials and especially for use in the process disclosed in U.S. Pat. No. 4,104,070 are polyalkyenyl phenols such as polyvinyl phenol of the type disclosed in U.S. Pat. No. 4,439,156. A major drawback in attempting to use these polymeric materials as photoresist materials is that photoresist systems prepared using these materials exhibit very high dissolution rates for development in alkali solutions normally used as developers. The very high dissolution rates prevent adequate control over processes to obtain the necessary image profiles, and in the case of low dimension configurations, e.g. 1 lines, such configurations are totally lost. Attempts to decrease the dissolution rate by substituting alkali solutions of relatively weak concentration are unsuccessful as the weakened solutions are depleted too quickly to be of use in the manufacture of devices.
A second drawback to the use of polyalkenyl phenols in the preparation of photoresist systems is that they soften and begin to flow at about 150.degree. C. The tendency to flow distorts the resist image pattern, and in the fine geometry patterns which are required, for example, in integrated circuit manufacture, the distortion can result in dimensional changes or even in the closing of fine lines. Such distortion can take place when the resist image is baked to improve the adhesion of the resist to the substrate or when the resist image is heated during the processing of the substrate such as by hot etchant solutions, ion implantation or plasma etching.
There is, therefore, a need in the art to control the dissolution rate and to increase the flow resistance of photoresist compositions in which a polyalkenyl phenol resin comprises the polymeric binder for the resist.